1. Field of the Invention
This invention relates generally to pressure measuring instruments for use in industrial process instrumentation systems. More particularly, this invention relates to pressure sensors of the type comprising a vibrating wire which is tensioned in proportion to the pressure being measured, and which vibrates at a frequency proportional to its tension.
2. Description of the Prior Art
It has been well known for many years that the vibratory frequency of a taut wire is a function of the tension in the wire. It was also early recognized that a pressure or force measuring instrument could be based on this relationship, by causing the wire to vibrate while tensioned with the unknown force applied thereto, and by detecting the frequency of vibration to produce a corresponding measurement signal. Considerable effort has been devoted to developing instruments in accordance with those principles.
In certain of the prior art instruments, the vibratory element was formed of magnetic material which was made to vibrate by applying thereto an alternating magnetic field as by means of a drive coil or the like; the vibratory movement of the element was detected by a pick-up coil. Such an approach also has been used with vibrating elements in the form of relatively stiff struts, as disclosed for example in U.S. Pat. No. 3,600,614.
In another approach to the problem, it has been proposed to position an electrically-conductive taut wire in a permanent magnetic field and to connect the ends of the wire to an electronic oscillator to develop an oscillating current in the wire so as to cause the wire to vibrate at its resonant frequency. The frequency of the oscillator represents the measurement signal. U.S. Pat. Nos. 2,455,021 and 3,543,585 show various arrangements of this sort.
In some instrument proposals, the vibrating wire was mounted within an enclosed chamber and one end of the wire was connected in tensioned state directly to a single pressure-responsive diaphragm which was axially-aligned with the wire. Pressure applied to the diaphragm was transmitted through to the wire to vary the wire tension in accordance with the diaphragm pressure. One instrument of that character, known as the Vibrotron Gage, was placed on the market a number of years ago by a major instrument manufacturer, but that instrument does not presently appear to be available commercially. U.S. Pat. No. 3,543,585 shows a later proposed instrument of similar character, but wherein the vibrating wire was connected to an axially-aligned bellows rather than a diaphragm.
Notwithstanding the extensive development efforts represented by the considerable prior art in this field, the instruments which have resulted have not been satisfactory for many important applications. In particular, the prior art instruments generally have been incapable of sufficiently accurate operation, especially over long periods of time, and under adverse environmental conditions such as shock and vibration, and variations in ambient temperature.